Rotational and Rotational-Vibrational Raman Spectroscopy of Air to Characterize Astronomical Spectrographs

Abstract

Raman scattering enables unforeseen uses for the laser guide-star system of the Very Large Telescope. Here, we present the observation of one up-link sodium laser beam acquired with the ESPRESSO spectrograph at a resolution λ /Δ λ ˜140 000 . In 900 s on source, we detect the pure rotational Raman lines of <SUP>16</SUP>O<SUB>2</SUB>, <SUP>14</SUP>N<SUB>2</SUB>, and <SUP>14</SUP>N <SUP>15</SUP>N (tentatively) up to rotational quantum numbers J of 27, 24, and 9, respectively. We detect the <SUP>16</SUP>O<SUB>2</SUB> fine-structure lines induced by the interaction of the electronic spin S and end-over-end rotational angular momentum N in the electronic ground state of this molecule up to N =9 . The same spectrum also reveals the ν<SUB>1 ←0</SUB> rotational-vibrational Q-branch for <SUP>16</SUP>O<SUB>2</SUB> and <SUP>14</SUP>N<SUB>2</SUB> 14. These observations demonstrate the potential of using laser guide-star systems as accurate calibration sources for characterizing new astronomical spectrographs.